A mix of both. The ambience music is great because it sets the whole world to have a special feeling. Actual ST would be mean to be used in certain decisive events (as the ending in YN, or some other important thing like that).
Also, keep the filesize with low pls.
Most of the Door's worlds in YN/fangames are giant empty maps with a 2deep4u background and dildos dancing around. That isn't interesting at all. I think >=12 doors is ok, but where these worlds are actual worlds on their own rather than copies of the original (snow/neon/forest/etc world).
The ending doesn't rule over what happened before it. You can speculate even knowing what happened in the end, because you don't know what happened before that, or what led to said ending.
However, if you're saying that you won't leave any black tile in the maze (Or, in other words, explain EVERYTHING), then at least make it like some kind of puzzle where connecting the dots in an interesting way leads to a firm base, though still leaving some empty spots for the player's imagination.
Kinda like, for example, you can show part of the whole image, and as you come closer to the ending, the picture becomes clearer, and also showing new points of view you may or may not have been capable of noticing before. I always love when in a game they start explaining all in the detail and you realize that you missed something that was there, in front of you, clear as water in broad daylight.
But that's just my opinion.